Some of the most significant problems in hydraulic and pneumatic control and power systems involve the transmission of fluid control signals and fluid power over appreciable distances. This is primarily the result of two inherent limitations of such systems relative to electrical control and power systems: the rate of fluid signal transmission is relatively low; and, the cross sectional area of the conduit or passage which transmits the fluid signal or working fluid is relatively great. These two factors are competing; the rate at which a conduit can transmit power and control signals improves as the cross sectional area is increased.
These constraints are not a major concern in many common applications of hydraulic and pneumatic control and power systems, such as in earth moving equipment, where fluid transmission distances are small and size and weight limitations are not a significant consideration. However, there are many applications in which these two limitations of fluid control and power systems impose significant economic and technical disadvantages. Perhaps the most challenging problems in this area are presented by hydraulic control systems for subsea control valves and other equipment situated at relatively great distances from a surface facility from which the valves are controlled.
In offshore oil and gas wells it is common to locate the plurality of control valves required for each well in a subsea tree situated at the seafloor. It is necessary to provide for control of these valves from a surface location, such as an offshore production platform. It has been found that hydraulic control systems are well suited for this purpose.
Existing hydraulic control systems for subsea wells fall into two basic classes, direct acting and indirect acting. In direct acting systems, a discrete hydraulic control line is provided for each hydraulic actuator. Application of hydraulic pressure to a selected control line serves to actuate the corresponding actuator. In the most basic of direct acting systems, a control line is connected directly to the valve actuator, with the hydraulic fluid in the control line serving as the working fluid to operate the subsea valve. In a refinement of the direct acting system, each control line operates a pilot valve. Actuation of the pilot valve connects the valve actuator to a separate source of pressurized working fluid which operates the valve actuator. Though simple and reliable, direct acting hydraulic control systems suffer the disadvantage of requiring a separate control line extending from the ocean surface to the submerged well for each control valve. Where there are a significant number of wells, a large number of control lines is required, yielding a control umbilical of large diameter. The cost of an umbilical having a great number of individual control lines can be substantial. Further, the use of a relatively large diameter umbilical poses especially great problems in deep water wells because of the high loading imposed on the umbilical, relative to a smaller diameter umbilical, in the course of installation. Also, providing means to resist the current and wave induced drag on the relatively large diameter umbilical can pose serious technical problems. Further, direct acting systems are often economically impractical for satellite wells which are located a great distance from the offshore platform from which they are controlled. Not only does the great length of the umbilical impose a significant expense, but anchoring the large diameter umbilical to restrain it from movement caused by current can also add significant cost.
In indirect acting hydraulic control systems, one or more hydraulic control lines are used to transmit coded signals to the wellhead. Typically, these control lines serve only to transmit signals and do not transmit working fluid for operation of the subsea valves. The coded signal transmitted by the control lines is received by a subsea switching valve. The switching valve addresses the subsea valve corresponding to the signal transmitted by the control lines. The use of coded signals avoids the need for a dedicated control line for each valve. This yields a decrease, relative to direct acting systems, in the number of hydraulic lines extending from the surface to the subsea well.
Pressure sequenced control is one of the most common types of indirect acting control systems. Indirect acting systems employ a number of pilot valves, each of which is adapted to operate in a set range of pressure levels. These pilot valves are connected in parallel to a single control line. By application of a selected pressure level to the control line, all of the pilot valves which are set to operate at that pressure level will operate. To ensure accurate operation of such systems, the pilot valve set points must be separated by about 2.8 MPa (400 psi). This limits the number of functions that can be controlled by any one control line. Details of one form of pressure sequenced control system are discussed in U.S. Pat. No. 3,993,100, issued Nov. 23, 1976.
A second type of indirect acting hydraulic control system which has been utilized in the control of subsea wells is disclosed in U.S. Pat. No. 4,356,841, issued Nov. 2, 1982. In this system, a control line extends from a surface control station to a subsea switching valve. The switching valve is adapted to assume a plurality of positions, in each of which it connects a corresponding one of a set of pilot valves to a source of fluid pressure. The switching valve operates through its sequence of positions in response to receiving pressure pulses via the control line. By applying a selected number of sequenced pulses to the control line, a corresponding valve actuator is operated. A disadvantage of this system is that where the length of the control line is great, as is often the case in subsea applications, an appreciable delay must be allowed following each pulse to ensure that the individual pulses remain discrete at the switching valve. This can impose significant delays in the operation of the subsea valves. This system is further disadvantageous in that the subsea valves must be operated in a set sequence.
It would be advantageous to provide a control system for subsea wells and other equipment requiring the control of multiple fluid actuated elements from a remote location in which only a relatively small number of control lines are required, which permits the fluid actuated elements to be operated in any desired sequence, and which is not dependent on the use of sequenced code signals.